1. Field of the Invention
The present invention is in the field of integrated circuits. More particularly, the present invention is in the field of integrated circuits of surface-mount technology type, having a package which houses the semiconductor chip and which provided electrical interface from this chip to plural electrical contacts disposed in an array on the bottom of the package. The chip is mounted to a circuit board, for example, by surface mounting its package in electrical connection with a congruent array of electrical contacts on the circuit board. Generally, a plurality of solder balls at the congruent contacts along with a solder reflow operation is used to both electrically connect the contacts of the integrated circuit package with the congruent contacts of the circuit board, and to physically mount the integrated circuit package on the circuit board.
2. Related Technology
Conventional surface-mount type of integrated circuits are known in which a ceramic package houses the integrated circuit, and which have a plurality of electrical contacts on the lower surface of the package. These electrical contacts are connected to congruent contacts of a circuit board by use of a corresponding plurality of solder balls depending from the contacts of the package. These solder balls are generally all of the same size. For example, the solder balls may be about 0.030 inches in diameter as set on the package bottom surface. The circuit board to which the package is to be mounted generally has a congruent array of electrical contacts at which a corresponding plurality of pads of solder paste (solder particles in paste flux) are applied. When the integrated circuit package is placed on the circuit board, the solder balls individually contact the pads of solder paste. Next, the circuit board and integrated circuit are placed in a solder reflow furnace, at a temperature and for a time sufficient to reflow the solder balls and to coalesce the solder particles of the solder paste into a unitary solder connection with the solder balls of the integrated circuit package.
Ceramic packages of the type described above generally are very stable and predictable dimensional. That is, the typical ceramic package may have an non-planarity of its bottom surface of no more than 0.001 to 0.002 inches across a typical package dimension of 1.125 to 1.5 inches square. Consequently, when the solder balls are placed on the bottom surface of such a package, they also define a mutual solder ball array plane which is non-planar by about the same amount as the package. That is, the solder balls themselves are generally made by a process similar to the making of buckshot, for example, which produces solder balls with a diameter dimension variation of no more than about .+-.0.002 inches, or by a screen printing process using solder paste which is then reflowed to produce solder balls with a slightly higher degree of dimensional variability. With all of the conventional methods of making and placing the solder balls on a package bottom surface, the combination of package and solder ball dimensional variation creates an non-planarity of about 0.004 to 0.005 inches at most. The conventional furnace solder reflow mounting and electrical connection method described above will achieve reliable mounting and electrical connections with this degree of dimensional variation. Such reliability in the mounting and electrical connection of the integrated circuit packages to their mounting boards is important because the solder joints between the contacts of the package and those of the circuit board are highly difficult to visually inspect non-destructively once the package is in place on the circuit board. Statistical methods of quality control along with destructive testing methods must be relied upon to provide confidence in the good physical securing of any particular package to its circuit board along with reliable electrical connection of each of the package contacts to its corresponding contact of the circuit board.
Conventional surface-mount type of integrated circuit packages are also known which are fabricated of plastic rather than ceramic material. These plastic packages for integrated circuits are advantageously less expensive to manufacture, and have other advantages over the ceramic packages described above. However, the plastic integrated circuit packages also have serious disadvantage because the plastic material of these packages is not as dimensional stable as the more conventional ceramic material of the packages described above. That is, during the manufacture of these plastic packages, which is generally accomplished with an injection molding or plug molding type of manufacturing, the base portion of the package may warp out of planarity. That is, the base portion of the package which carries the depending solder balls may not be flat. If this base portion of the integrated circuit package is bowed or wavy, for example, then the solder balls depending from this surface will define cooperatively a solder ball contact array plane which is also bowed or wavy.
By careful consideration of the events taking place during the solder reflow operation to mount and electrically connect such a package to a circuit board, an appreciation of the problem outlined above may be gained. When an integrated circuit package with its depending array of solder balls is placed on a circuit board, which has been prepared with pads of solder paste (solder particles in flux) on the contacts of the circuit board, if the package and circuit board are sufficiently planar, then each of the solder balls will contact its corresponding pad of solder paste. Subsequently, when the circuit board and circuit package are heated in the solder reflow furnace, the reflowed solder balls and coalescing solder paste will join by mutual surface tension.
However, if the integrated circuit package is sufficiently bowed or wavy that some of the solder balls do not contact their corresponding solder paste contact pads, then mutual surface tension is not developed at these locations. When the solder paste and solder balls which are not contacting one another are reflowed, the surface tension of each tends to make them flatten out along their respective surfaces, and actually draw away from one another. Consequently, an electrical connection is generally not effected at the solder balls which do not contact their corresponding pads of solder paste. Also, the solder balls which do not electrically contact their corresponding circuit board contact do not participate in the physical securing of the package to the circuit board. Moreover, the physical mounting of the package to the circuit board is also compromised when solder balls do not join with their corresponding solder paste pads.
A contemporary plastic integrated circuit package may have a bottom surface non-planarity of about 0.004 inches. When solder balls are added on this bottom surface, and contribute their own dimensional variability of 0.002 inches, or slightly more, then the total non-planarity to be expected at the solder ball array plane is at least 0.006 inches. This non-planarity is at the limit of what can be tolerated, and in fact statistically decreases the reliability of the electrical connections between such a package and a circuit board upon which it is to be mounted. In fact, a joint industry council, JEDEC, has proposed an industry standard for the planarity of the solder ball contact array at the bottom surface of plastic integrated circuit packages to address this problem. This standard calls for a non-planarity of no more than 0.006 inches.